Process for recovery of spent etchant

ABSTRACT

A chelation process and apparatus for treating an acid solution, namely nitric acid photoengraving etchant, having a high concentration of magnesium and having a heavy metal contaminant produced by zinc leached during etching. The process requires adjusting pH of a batch quantity of the etch solution to a value permitting chelating to occur and mixing carbamic sodium salt hydrate chelating agent with the solution in quantity sufficient to cause precipitation from the solution of a carbamic salt compound precipitate of the contaminant, and separating the precipitate from the remaining solution by filter press for removal from the solution for safe environmental disposal. The remaining liquor is safely sewered. The apparatus is mobile for use on photoengraving premises.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of aqueous effluents and, moreparticularly, to a process for treatment of spent acid etch solution,particularly acid solutions having heavy-metal contaminants by reductionof the heavy metal zinc from such etchant to levels safe for discharge.

As regulatory rules for waste stream effluent become more stringent, amethod to remove heavy metals becomes mandatory. Currently, magnesiumphotoengraving plate is etched in a solution containing nitric acid andan organic additive. Typically, the magnesium sheet contains by weightthree percent (3%) aluminum and one percent (1%) zinc with otherimpurities being less than 0.05 percent (0.05%). As the magnesium sheetis processed in the etch bath, aluminum, zinc, magnesium and minorimpurities are leached into the acid bath. The spent acid bath generallycontains 50,000 ppm magnesium and 500 ppm zinc. While a locality mayallow a discharge of 5 ppm zinc, national or other governmentalregulations may establish a discharge limit which is more restrictive,e.g., less than 2 ppm.

An acceptable practice, currently in use at many locations, for removingzinc ions from the etch solution involves the addition of sodiumhydroxide. The hydroxide ion neutralizes the acid and, by raising the pHabove 9.2, insoluble zinc hydroxide is formed. Solids can be removedfrom a hydroxide treated bath by either settling and decanting thesupernant (aqueous layer without solids) or by filtration. Such ahydroxide precipitation method of zinc removal is complicated bysimultaneous precipitation of most of the dissolved magnesium, which isnot a target metal, thereby adding significantly and undesirably to thevolume of solids of which are to be disposed.

SUMMARY OF THE INVENTION

The use of sodium dimethyldithiocarbamate (DTC) in accordance with thepresent invention, as proposed herein, may be used to reduce the volumeof solids to be disposed of while simultaneously reducing the zinc levelbelow 2 ppm in this nitric acid etchant. The mechanism by which such achelating agent accomplishes this is by specifically chelating with thefamily of metal ions (zinc [II], silver [I], copper [II], lead [II],cadmium [II], manganese [II]) at low pH values. By this controlledprecipitation, the magnesium ions are left virtually untouched in theetchant solution. The filtration of the solids from the DTC etchant andthe neutralization of etchant solution to accommodate disposal levelshave created a disposal system in which material, upon being analyzed,is found to be suitable for disposal in public landfills.

Accordingly, it is among the objects of the invention to provide animproved process for treatment of zinc-contaminated acid solution,specifically spent etchant, and specifically such a process capable ofseparating a heavy metal contaminant from such an acid solution, i.e.,from etchant in such a way as to precipitate the zinc from the etchantby chelating, to levels suitable for disposal in public landfills.

It is also an object of the invention to provide an improved system,including portable apparatus, for carrying out such a process.

It is a further object of the invention to provide such a system andprocess capable of reducing zinc concentrations in spent etchant tolevels typically required by governmental regulations, and even tolevels of less than 2 ppm.

It is a further object of the invention to provide such a system andprocess which, as utilized to treat spent etchant of the typespecifically used for etching of magnesium sheet which includes aluminumand zinc; and which obviates separate removal of dissolved magnesium,thereby allowing safe discharge of an aqueous solution of magnesium andother non-objectional impurities, while safely lowering zincconcentrations to extremely low levels. A further object of theinvention is to provide a process and system for safe heavy metalcontainment removal from an acid solution which allows use of a singlechelating agent for both removal of the contaminant and neutralizationof the solution.

Briefly, the foregoing objects are achieved by a process for treating anacid solution having a heavy metal contaminant to remove the contaminantfrom the solution for safe environmental disposal. The process adjuststhe pH of a quantity of the solution to a value permitting chelating tooccur and involves mixing carbamic sodium salt hydrate chelating agentwith the solution in quantity sufficient to cause complete precipitationof the contaminant from the solution as a carbamic salt compoundprecipitate of the contaminant. As a result, the precipitate becomesseparated from the remaining solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process according to the presentinvention.

FIG. 2 is a simplified schematic diagram of mobile apparatus asconnected with photoengraving etching components to provide a systemcarrying out the process of FIG. 1.

GENERAL DESCRIPTION OF METHODOLOGY

For photo-engraving of magnesium sheet, etching thereof is carried outin an etching solution of 14%-22% (by weight) nitric acid with a filmingagent additive. Typically, the magnesium sheet contains 1% zinc which,during the developmental process, leaches into the etchant bath. Theetch solution is considered exhausted or "spent" etch solution when itcontains 50,000 ppm magnesium and 500 ppm zinc.

Referring now to the drawings, a process of the invention is illustratedin the form of the process flow diagram of FIG. 1.

The general methodology of FIG. 1, and pH levels set forth therein areexemplary and illustrative only.

A spent etchant solution such as used for having etched such magnesiumphotoengraving plates typically contains 3% by weight of aluminum and 1%by weight of zinc with other impurities in negligible concentrations isprovided to the system at 10.

For treatment according to the invention, the spent etchant istransferred, as at 12 into a processing vessel for treatment.

At 14, a basic solution is added to the processing vessel in such anamount so as to achieve a predetermined pH level, that is, to a valuepermitting chelating of the zinc to occur by use of a carbamic sodiumsalt hydrate. The pH as a minimum should be 2.5, more preferably atleast be 3.5 and most preferably a pH of at least 4 (but not more thanabove 6), as shown at step 16. If the pH has not yet reached 4, forexample as indicated at process step 18, additional basic solution isadded until the pH level is 4. At step 20, a selected carbamic sodiumsalt hydrate chelating agent in accordance with the invention is thenadded to the processing tank in at least stoichiometric amount,preferably up to about 15% more, for causing substantially completechelation of the zinc, as a typical sulfide precipitated metal ion, byprecipitation from the solution as a carbamic salt compound zincprecipitate. For this purpose, the chelating agent is stirred into thesolution. Zinc ions are, of course, held in suspension in the processingtank. Stirring may be carried out continuously over a predeterminedinterval.

Then, at step 22, the pH of the solution is again adjusted to apredetermined level corresponding to predetermined effluent limits forpH, e.g., of 6, as by adding additional sodium hydroxide solution. Ifthe pH is not yet up to a level of 6, for example, as shown at step 24,additional basic solution is added, and further stirring take place.When the pH level is at the predetermined level, a preselected quantityof a flocculating or filtering aid, e.g., diatomaceous earth, is addedto the pH-adjusted solution, as at step 26. The suspended solution isthen transferred at step 28 to a filter press.

Using such a filter press, the aqueous solution is filtered to removesolids by retaining the suspended solids in the filter cake. Aftercycling, the filter press is checked at 30 to determine if a solid cakehas resulted from the suspended solids. If not, a further cycling iscarried out as shown at 32 until a solid cake results.

After the filter cake has reached a predetermined density and size, theliquor is discharged, as at 34, from the process and sewered orotherwise suitably disposed of as at step 36. The filter cake of zinccompound of the carbamate salt dries to a flake form, and then istransferred, as at 38, to a landfill.

DETAILED PREFERRED PROCESS DESCRIPTION

Etchant for such photoengraving conventionally consists of a 14% to 22%nitric acid solution with a 5% by weight oil additive or filming agent.During the processing this solution is sprayed on the magnesium plateand etches away magnesium alloy not protected by the photoresistcompound. It is at this period that metallic ions are leached into theetching solution. The typical composition of the magnesium sheet used inthis process is found in Table 1.

                  TABLE 1                                                         ______________________________________                                        Percent chemical composition of photoengraving                                magnesium sheet.                                                                      Al     Zn    Ca        Ni    Fe                                       ______________________________________                                        Nominal   3.1      1.0   0.02    0.0007                                                                              0.008                                  Upper limit                                                                             4.0      1.6   0.04    0.0050                                                                              0.010                                  Lower limit                                                                             2.5      0.7   0.0     --    --                                     ______________________________________                                    

The etchant bath, strongly a nitric acid solution having a pH typically0.5-1, but possibly approaching zero, may be considered exhausted whenits magnesium content as determined by weight loss measurement reaches1.5 ounces magnesium per liter of solution. Corresponding zinc levels inthis solution usually exceed 250 ppm and typically range up to 500 ppm.Also present in the spent acid solution in an appreciable quantity isaluminum. Copper ions are typically less than 0.05% by weight. Toprocess a spent acid etch solution, the etch solution is transferred tothe reaction vessel via a chemical transfer pump capable of withstandingextremely low pH solutions, or by siphoning. On completion of thistransfer, power is supplied to a mixer for the vessel and pH controllerfitted to the vessel as later described.

A solution of neutralizing material is fed into the reaction vessel,while the spent acid solution is stirred by the mixer, until apredetermined pH 4 is obtained. Such pH may be in the generallypreferred range of from 2.5 to 6, and a pH of 4 being most specificallypreferred.

The neutralization solution may be selected from the group consisting ofsodium hydroxide, sodium acetate and magnesium hydroxide, as well asmixtures thereof, such as a mixture of a 50% solution of sodiumhydroxide or a 50% solution of sodium acetate. Then a 40% to 50%solution of dimethyldithio-carbamic acid (dimethyldithiocarbamate)("DTC"), a sodium salt trihydrate having the formula (C₂ H₅)₂ NCS₂ Na .3H₂ O, is added to the reaction vessel according to the followingspecifically preferred criteria, based upon an assumed zincconcentration of 0.5 g zinc/liter of solution:

    liters DTC=(0.0075 liters DTC/liters etch solution)×(liters of etch solution)                                                 (Formula 1)

Formula 1 provides for addition of DTC in amount based on such a zinclevel, and for an additional amount of about 10% by weight. At least astoichiometric amount of DTC is added to achieve salient objects of theinvention.

The etch solution and added DTC, are allowed to react for at leastone-half hour and preferably about one hour, after which time the DTCwill have chelated the aqueous zinc and the pH value will have risenfrom 4.0 to about 7.0. If the pH is less than 6.0, an additional amountof either sodium hydroxide (or an alternative neutralizing solutionbeing used for the treatment process) or DTC must be added to adjust thepH to a minimum of 6.0. If additional DTC is added, the mixture must beagain stirred for one hour. The pH is then finally adjusted with base oracid to achieve a final pH disposal value, namely a minimum pH of atleast 6, and not more than a pH of about 7.

Diatomaceous earth is added to the reaction vessel as a filtering aidaccording to the following formula (based upon an assumed zincconcentration of 0.5 grams per liter and 130 L. of etch solution):

    (lbs. diatomaceous earth)=(0.045 lbs. diatomaceous earth/liters) ×(liters of etch solution treated)                  (Formula 2)

An alternative filtering aid can be an anionic polymer. An example ispolyacrylamide.

The resultant slurry is allowed to mix for at least about 5 minutesbefore starting a cycle of the filter press (28). The filter press pumpis started at 25 psi until pump cycle time has averaged out to about 0.5min. Pressure is in increased 25 psi increments each time the pump cycletime has averaged out to about 0.5 min. until a total pressure of 100psi is attained. When pump cycle time is one minute at 100 psi, thefilter press cycle is considered finished. Air to the filter press pumpis turned off and an air blowdown on filter press is performed at 40 psifor 10 to 15 minutes. The filter press is allowed to equalize tobarometric air pressure before opening. The press is opened and thefilter cake, believed to be bis(methylcarbamodithioato-S,S')zinc, isremoved for disposal in approved landfill or as appropriate.

Alternatively, the pH of the solution may be raised by direct use ofDTC, a strong base, without resort to a separate neutralizing solution,at least to the minimum value of about 2.5 which permits chelation tooccur, but more preferably at least 3.5, and most preferably 4, as byadding at the outset at least to a stoichiometric amount of the DTC,which will cause corresponding increase in pH; additional DTC may thenbe added to raise the pH still further to the preferred minimum; andultimately to the safe pH level for discharge of the liquor.

Before allowing effluent to be discharged, outflow from filter press ischecked for zinc concentration using prescribed methods for zincdetermination, and the effluent, being an environmentally benign liquorin the form of a magnesium solution at safe pH levels, may be seweredsafely.

The following examples illustrate variations in the above-describedmethodology in accordance with the invention.

EXAMPLE 1

A test quantity of 500 ml of nitric acid etch solution containing 12,000ppm magnesium, 400 ppm aluminum and 120 ppm zinc is treated by the aboveprocess. After raising the pH to 4 with a 50% sodium hydroxide solution,7.5 mls of a 45% solution of DTC are added to the solution and stirredfor one hour and then filtered. The analysis of the remaining solutionfor dissolved metals shows 11,500 ppm magnesium, 49 ppm aluminum and 0.9ppm zinc. The amount of suspended solids increases during treatment from445 ppm to 4650 ppm, as increased by filtration before and after. Theresulting effluent thus has a zinc concentration well withingovernmental guidelines of 2.7 ppm maximum for effluent.

EXAMPLE 2

A batch quantity of 130 liters of an exhausted etch solution, namely awhole exhausted etchant bath, containing 55,000 ppm magnesium, 630 ppmaluminum and 559 ppm zinc, is treated with a 50% solution of sodiumhydroxide until a pH of 4 is obtained. Two liters of a 45% solution ofDTC, corresponding to about 400 g of DTC per liter of DTC solution, areadded to the solution and stirred for one hour. A rationale is that ifthere is zinc concentration of 500 ppm in the spent etchant, 6 ml (±5%)of the DTC solution is added per liter of etch solution to be treated.The analysis of this filtered solution for dissolved metals shows 50,000ppm magnesium, 78 ppm aluminum and 1.4 ppm zinc. To determine ifadditional zinc can be precipitated, an additional liter of the 45%solution of DTC is added, stirred, filtered and analyzed. 50,000 ppmmagnesium, 48 ppm aluminum and 1.2 ppm zinc are found, therebydemonstrating effective precipitation in the first reaction based uponthe rationale.

EXAMPLE 3

In this example, 500 ml of etch solution containing 12,000 ppmmagnesium, 400 ppm aluminum and 120 ppm zinc are first treated with a50% solution of sodium acetate to a pH of 4.5. Then 3.6 ml of a 45%solution of DTC are added and the mixture is allowed to stir for onehour. The analysis of the filtered solution shows approximately 12,000ppm magnesium, 375 ppm aluminum and 1.2 ppm zinc. Thus, use of sodiumacetate as an alternative buffer allows selective chelating of zincwithout precipitating aluminum as aluminum hydroxide.

EXAMPLE 4

Without first being neutralized as in Examples 2 and 3, 500 ml etchsolution containing 12,000 ppm magnesium, 400 ppm aluminum and 120 ppmzinc are treated with a 329 ml of a 45% DTC solution to a pH of 6,wherein the DTC, a strong base, is used as a combinative agent for bothneutralizing the etch solution and chelating. Upon analysis, magnesiumand aluminum concentrations are virtually unchanged from Example 3 whiledissolved zinc is not detected.

EXAMPLE 5

Under conditions like Examples 1 and 2, 131 L. of an exhausted etchsolution containing 55,000 ppm zinc are treated with a 50% solution ofsodium hydroxide to a pH of 4. Then 3 liters of a 45% solution of DTCare added to the reaction vessel and stirred for one hour. To minimizefilter press clogging by very fine particles of precipitated zinccarbonate, diatomaceous earth is added as a filtering aid in thegenerally preferred amount of from about 5 to about 20 lb/100 gal. ofetch solution, and more preferably in the range of 5-15 lb/100 gal. ofetch solution, and most specifically preferred the amount of 6 lb/100gal. of etch solution. The resultant slurry is stirred for 20 minutes,then filter pressed starting at 25 psi and increasing the pressure in 25psi increments to 100 psi. The resultant filter cake contains zinccarbamate precipitate with diatomaceous earth. The total cake solids isfound to be 51.8% (by weight) with a specific gravity of 1.17 and a wetcake density of 73 lbs/cu.ft. The effluent contains 2.1 ppm zinc. Thefilter press is a gasketed plate press, total volume of 0.3 cu.ft.Feeding the filter press is a double diaphragm pump. The reaction vesselis a polyethylene 100 gal. tank with a 1/4 HP, 4" single prop mixer. pHcontrol is controlled with a pH controller which activates a chemicalfeed pump for caustic addition.

EXAMPLE 6

Conditions as in Example 5 are carried out but substituted fordiatomaceous earth as a filtering aid is an anionic polymer, namelypolyacrylamide in the ranges of lb./100 gal. of etchant, as follows:

Generally preferred: about 0.001 lb. to about 0.0025 lb.

Specifically preferred: 0.0012 lb. to 0.0018 lb.

Most preferred: 0.0013 lb.

At the most preferred amount usage, zinc concentration in the effluentis 2.1 ppm.

EXAMPLE 7

A test quantity of 500 ml of nitric acid etch solution containing 12,000ppm magnesium, 400 ppm aluminum and 120 ppm zinc is treated by the aboveprocess. After raising the pH to 4 with a 50% sodium hydroxide solution,7.5 mls of a 45% solution of sodium diethyldithiocarbamate are added tothe solution and stirred for one hour and then filtered. The analysis ofthe remaining solution for dissolved metals shows 11,500 ppm magnesium,49 ppm aluminum and 0.9 ppm zinc. The amount of suspended solidsincreases during treatment from 445 ppm to 4650 ppm, as increased byfiltration before and after. The resulting effluent thus has a zincconcentration of 2.7 ppm maximum, well within governmental guidelinesfor effluent.

Mobile Apparatus of System

Referring to FIG. 2, a system apparatus 100 for etch solution treatmentaccording to the above-described methodology comprises a mobile cart 102shown in schematic, highly simplified plan view. The cart may be ofsuitable metal or polymer-based structure providing a platform 104 withfloor casters (as shown in outline at 105) or the like at its cornersfor mobile use in a lithography laboratory or shop. In such premises,there are one or more etching apparatus, as at 106, providing lithographplate etching stations, each holding acid solution in a vessel inquantities of 100-150 liters.

Mounted suitably on platform 104 are a reaction vessel, i.e. processingvessel, 108 of the type described in Example 5, equipped with a stirrer100, and fitted for receiving spent etchant from the etching apparatus106, as by a hose 112, for which purpose a suitable pump (not shown)preferably is provided.

On cart 102 are provided a pH controller 114 of the previously describedtype adapted for sensing the pH of solution in vessel 108 and forautomatically selectively energizing and deenergizing a pump 116 for asa function of the sensed pH for transferring neutralization solutionfrom a suitable storage tank 118 (e.g., of polyethylene) as a functionof the pH sensed in the solution in vessel 108. Also provided on cart102 is a filter press 120 of the type noted in Example 5, for receivingtreated solution from vessel 108.

For simplicity, associated valves, plumbing, and other conventionalcomponents are not shown.

On the etching premises, a supply 122 of the preciptation, e.g.,chelating, solution for the process is maintained, as is a supply 124 ofthe filter aid described in example 5.

The cart, being mobile, may be relocated on premises to a positionproximate an etch machine 108 for treatment of its spent etchantaccording to the foregoing description. After treatment of the etchantas described hereinabove, the precipitate cake is transferred tolandfill and the environmentally safe liquor is sewered.

Conclusion

Treatment of spent magnesium photoengraving etchant with a carbamicsodium salt hydrate chelating agent such as DTC(dimethyldithiocarbamate) has been found to be effective in removingzinc from the acid etch bath. Zinc dimethyldithiocarbamate isprecipitated or complexed at low pH and removed from the acid etchsolution while allowing other innocuous metal ions, primarily magnesium,to pass through the filter press in solution. The most effective pH forsuch use of DTC has been found to be in the generally preferred range offrom about 4 to about 6. While DTC is a fairly strong base and can beused per se to neutralize the acid solution, it is economically morefeasible to use sodium hydroxide or sodium acetate to raise the pH ofthe etch solution before making DTC addition.

In view of the foregoing, it will be seen that the several objects ofthe invention are achieved and other advantages are attained.

Although the foregoing includes a description of the best modecontemplated for carrying out the invention, various modifications arecontemplated.

As various modifications could be made in the constructions and methodsherein described and illustrated without departing from the scope of theinvention, it is intended that all matter contained in the foregoingdescription or shown in the accompanying drawings shall be interpretedas illustrative rather than limiting.

What is claimed is:
 1. A process for treating a spent acid solutionwhich is a spent nitric acid photoengraving etchant used for etching ofmagnesium alloy photoengraving plates containing magnesium, aluminum andzinc, and the etchant is contaminated by zinc, to selectively remove thezinc from the solution for safe environmental disposal, comprisingadjusting the pH of the solution to not less than 2.5 but not exceeding6; adding to the solution a carbamic sodium salt hydrate precipitatingagent selected from the group consisting of diethyldithiocarbamic acidsodium salt and dimethyldithiocarbamic acid sodium salt, saidprecipitating agent being added in quantity sufficient to completelyprecipitate the zinc, stirring the solution with said precipitatingagent for a period to cause precipitation from the solution of acarbamic salt compound precipitate of zinc; and separating theprecipitate from the remaining solution as a waste material, wherebynonprecipitated amounts of said magnesium and aluminum remain insolution.
 2. A treatment process according to claim 1 wherein theprecipitating agent is dimethyldithiocarbamic acid sodium salt and theprecipitate is bis(dimethylcarbamodithioato-S, S') zinc.
 3. A treatmentprocess according to claim 1 wherein the step of adjusting the pH of thesolution is effected at least in part by adding said precipitatingagent.
 4. A treatment process according to claim 1 wherein the step ofadjusting the pH of the solution is effected by adding a basic substancebefore adding said precipitating agent.
 5. A treatment process accordingto claim 4 wherein the basic substance is a solution selected from thegroup consisting of sodium hydroxide, sodium acetate and magnesiumoxide.
 6. A treatment process according to claim 1 wherein the pH of thesolution is adjusted to not less than 3.5.
 7. A treatment processaccording to claim 1 wherein the pH of the solution is adjusted to notless than
 4. 8. A treatment process according to claim 1 wherein thestep of separation of the precipitate from the solution is effected byusing a filter press to convert the precipitate waste material into afilter cake suitable for transfer to a landfill.
 9. A treatment processaccording to claim 8 wherein before using said filter press to convertthe precipitate, the pH of the solution in further adjusted to a finalpH value safe for sewering of the solution after using said filterpress.
 10. A treatment process according to claim 8 wherein before usingsaid filter press to convert the precipitate, a filtering aid is addedto the solution as a flocculent.
 11. A treatment process according toclaim 10 wherein the filtering aid is diatomaceous earth.
 12. Atreatment process according to claim 10 wherein the filtering aid is ananionic polymer.
 13. A process for environmentally treatment spent acidetchant solution which is a spent nitric acid photoengraving etchantused for etching of magnesium alloy photoengraving plates containingmagnesium, aluminum and zinc, and the etchant is contaminated by zincresulting from said etching, to selectively remove the zinc from thesolution for safe environmental disposal of the etchant and the zinc,comprising adjusting the pH of the solution to not less than 2.5 but notexceeding 6; adding to the solution carbamic sodium salt hydrateprecipitating agent selected from the group consisting ofdiethyldithiocarbamic acid sodium salt and dimethyldithiocarbamic acidsodium salt, said precipitating agent being added in at leaststoichiometric quantity for causing substantially complete precipitationof the zinc, stirring the solution with said precipitating agent for aperiod for causing precipitation from the solution of a carbamic saltcompound precipitate of zinc; and separating the precipitate from theremaining solution as a waste material, whereby nonprecipitated amountsof said magnesium and aluminum remain in solution.
 14. A treatmentprocess according to claim 13 wherein the precipitating agent indimethyldithiocarbamic acid sodium salt and the precipitate inbis(dimethylcarbamodithioato-S, S') zinc.
 15. A treatment processaccording to claim 13 wherein the step of adjusting the pH of thesolution is effected at least in part by adding said precipitatingagent.
 16. A treatment process according to claim 13 wherein the step ofadjusting the pH of the solution is effected by adding a basic substanceto the solution.
 17. A treatment process according to claim 16 whereinthe basic substance is a solution selected from the group consisting ofsodium hydroxide, sodium acetate and magnesium oxide.
 18. A treatmentprocess according to claim 13 wherein the pH of the solution is adjustedto not less than 3.5.
 19. A treatment process according to claim 13wherein the pH of the solution is adjusted to not less than
 4. 20. Atreatment process according to claim 13 wherein the step of separationof the precipitate from the solution is effected by using a filter pressto convert the precipitate into a filter cake suitable for transfer to alandfill.
 21. A treatment process according to claim 20 wherein beforeusing said filter press to convert the precipitate, the pH of thesolution is further adjusted to a final predetermined pH value of from 6to 7, and after using said filter press, the solution is sewered.